1. Field of the Invention
The present invention generally relates to sheet piles which are driven through soils to form retaining walls or bulkheads. More specifically, the present invention relates to a reduced skin friction sheet pile which minimizes the downward frictional force (down drag) applied to the sheet pile by the settling of compressible soils surrounding the pile after the pile has been driven.
2. Description of the Related Art
Sheet piles are piles that interlock with each other to form walls or bulkheads that are capable of retaining soils and/or water depending on the particular application. Typically, these piles are relatively long, slender structural members which can be formed with any one of a variety of cross-sectional shapes including flat, U-shaped, and Z-shaped configurations, among others. The side edges of the sheet piles are configured to interlock with each other so that the piles can be arranged in edge-to-edge relationship to form bulkheads of various shapes. The sheet piles usually are driven through compressible soils to a sufficient depth so that the downwardly driven ends of the sheet piles penetrate harder underlying materials to a specified depth or penetration or until refusal is achieved. Typically, significant support of the piles is provided by these underlying harder materials.
Pile penetration results in a downward frictional force on the piles that is a function of the horizontal stress applied to the pile by the soil and the coefficient of friction of the pile's exterior surfaces relative to the soil. This downward force can result in failure of a sheet pile bulkhead due to an unexpected downward movement of the piles, or increase the time required to install the pile.
For many years, departments of transportation, structural engineers and geotechnical engineers have struggled with the problem of how to reduce downward frictional forces imposed upon piles. Many costly measures have been implemented to address this problem.
Sheet piles are installed without consideration for the downward friction force as part of the pile design. This method results in considerable increased construction cost.
In cases in which the anticipated structural load on the piles is increased to account for a downward frictional force anticipated, this results in a higher capacity pile which requires driving the pile farther into the harder consistency soils, thereby requiring an increase in pile length and an increase in the capacity of the pile driving hammer capable of driving a pile to a higher criteria. In some cases, these requirements increase the cost of pile driving and the length of time for pile installation and may require an increased cross sectional area of the pile to allow for the higher capacity.
Historically frictional considerations have not been an issue when installing sheet pilings. They have generally been overdriven which usually results in substantial added cost of installation.
Sheet piles oftentimes are temporarily driven into soils and then removed when associated work has been completed, such as when a temporary bulkhead for diverting water is required, or for accommodation of staged construction. As sheet piles are installed, sometimes cohesive soils are encountered and it becomes difficult to penetrate and friction becomes a major problem. The piles are difficult to install causing a tremendous drain on labor and equipment resources. During removal of the piles, frictional forces between the outer surface of the piles and the soils surrounding the piles increases the extraction force which must be exerted upon the piles in order to pull the piles form the soils. Depending upon the particular characteristics of the surrounding soil, extraction of such a temporarily driven pile can be extremely difficult, and can place excessive strain upon the pile removal equipment and involves additional time spent at the construction site.
Therefore, there is a need for improved sheet piles which address these and other shortcomings of the prior art.